docs(docs): add Approach and Explanation sections with mermaid diagrams to problem pages

apps/docs/content/(hash-table)/567-permutation-in-string.mdx

@@ -22,6 +22,24 @@ sidebar:
 - `1 <= s1.length, s2.length <= 10^4`
 - `s1` and `s2` consist of lowercase English letters.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["checkInclusion(s1, s2)"]) --> G{"k = len(s1) > len(s2)?"}
+  G -- yes --> X(["return False"])
+  G -- no --> I["need = Counter(s1), window = Counter()"]
+  I --> L{"more (right, c) in s2?"}
+  L -- no --> E(["return False"])
+  L -- yes --> A["window[c] += 1"]
+  A --> B{"right >= k? — window is now longer than k"}
+  B -- yes --> C["drop s2[right - k] from window, deleting the key at 0"]
+  B -- no --> Q
+  C --> Q{"window == need?"}
+  Q -- yes --> R(["return True"])
+  Q -- no --> L
+```
+
 ## Solution
 
 ```py
@@ -49,3 +67,7 @@ class Solution:
 
         return False
 ```
+
+## Explanation
+
+<YouTube id="UbyhOgBN834" title="Permutation in String - Leetcode 567 - Python" />